In recent years, in order to perform a CT examination with low exposure dose, an X-ray CT apparatus for executing image reconstruction using a successive approximation method has been developed. Through the image reconstruction using the successive approximation method, a CT image with less noise is obtained even at low dose.
NPL 1 discloses successive approximation projection data correction processing that is one of the successive approximation methods. The successive approximation projection data correction processing is one of the projection data correction processing that is the pre-processing of image reconstruction. In the successive approximation projection data correction processing, update equation having the projection value of projection data as a variable is used. The update equation includes a smoothing coefficient (also referred to as a correction coefficient or a penalty term) indicating the strength of correction. In the successive approximation projection data correction processing, the projection value is iteratively updated using the above-described update equation.